Those skilled in the arts of antenna arrays and beamformers know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. Small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers,” and a beamformer includes a beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different beams may be generated simultaneously.
Radar systems often use multiple antenna beams for tracking of disparate targets, and sometimes for tracking single targets. One scheme for use of multiple beams involves monopulse techniques, in which angle tracking information is obtained from multiple beams, ideally with but a single transmitted pulse. Monopulse operation is accomplished by generating two, or more usually three, antenna beams, so that the simultaneously received echoes from the multiple beams can be compared. The usual monopulse beams are a sum (Σ) beam, and azimuth (Az) and elevation (El) difference (Δ) beams. Monopulse systems are described in many publications, as for example in U.S. Pat. No. 5,017,927 issued May 21, 1991 in the name of Agrawal et al. Agrawal et al. in one arrangement uses three separate beamformers, namely Σ, Az Δ, and El Δ beamformers, to generate the three different beams. These beamformers can be manifested in an array of a plurality of elevation Σ, Az Δ, and El Δ column beamformers which connect to the antenna elements, and an array of azimuth Σ, Az Δ, and El Δ row beamformers, which connect the Σ, Az Δ, and El Δ ports to the column beamformers.
FIG. 1 is a representation of a prior-art array antenna as described in the abovementioned Agrawal et al. patent. As described therein in FIG. 1, radar system 10 includes an antenna array 12 including individual antennas or antenna elements 141, 142, 143, . . . 14N-2, 14N-1, and 124N arrayed in a column designated 161. Other columns 162, 163 . . . 16N are illustrated in a general manner as being located behind column 161, so as to form a two-dimensional rectangular array of antenna elements.
Each antenna element 141, 142 . . . 14N of columns 161, 162, . . . 16N of antenna array 12 of FIG. 1 is associated with a phase shifter 18. For example, elemental antenna 141 of column 161 is associated with a phase shifter 181. Similarly, each of the elemental antennas 142, 143 . . . 14N of column 161 are associated with a phase shifter 182, 183 . . . 18N. As also illustrated in FIG. 1, phase shifter 181 has an output transmission line (cable) 201 which, together with output cable 20N of phase shifter 18N of column 161, is connected to a sum-and-difference hybrid circuit 221. Each of cables 201 and 20N is connected to a separate input port (input) of hybrid circuit 221. It will be noted that phase shifters 181 and 18N are associated with elemental antennas 141 and 14N, the first and last (top and bottom) antenna elements of column 161. Similarly, the output of phase shifter 182 is coupled by way of a cable 202 to a second sum-and-difference hybrid splitter 222, together with the output from phase shifter 18N-1, coupled by way of a cable 20N-1. Phase shifter 182 is associated with antenna element 142, the second antenna element, and phase shifter 18N-1 is associated with penultimate antenna element 14N-2. A third sum-and-difference hybrid combining arrangement 223 receives inputs from the third antenna element 143 and its phase shifter 183 by way of cable 203, and from antepenultimate antenna element 14N-2 and its phase shifter 18N-2 by way of cable 20N-2, respectively. It can be seen that the outputs of the antenna elements of column 161 and their phase shifters are taken in pairs symmetrically disposed above and below the center of column 161, and the antenna outputs are combined in an array of sum-and-difference hybrids. The combination or array of sum-and-difference hybrids 22 associated with column 161 is designated 241.
Each of the other columns of FIG. 1, such as column 162, 163 . . . 16N, includes (not illustrated) its own column array of antenna elements 14 and phase shifters 18, each of which is associated with an antenna 14. Each of the other columns is also associated with an array 24 (not illustrated) of sum-and-difference hybrids 22. Only antenna array column 16N is illustrated in FIG. 1 as being connected by cables 20 to its associated sum-and-difference hybrid array 24N.
In the arrangement of FIG. 1, the sum output produced at the upper output of hybrid 221 of hybrid array 241, is coupled by way of a cable 261 to an input of a sum combiner or beamformer 301. Similarly, the upper or sum (Σ) outputs of sum-and-difference hybrids 222 and 223, and all the other hybrids (not illustrated) of hybrid array 241, are coupled by a cable 26 to sum combiner 301, which combines the sum signals, and which couples the combined sum signals to a single output cable 341. Similarly, the difference (Δ) output ports of sum-and-difference hybrids 221, 222, 223, . . . 22n/2 of hybrid array 241 of FIG. 1 are each connected by way of a transmission line 28 to separate inputs of a difference combiner or beamformer 321. Thus, the Δ (lower) output port of hybrid 221 is connected by way of a cable 281 to a first input of Δ combiner 321, the a output port of hybrid 222 is coupled by way of a cable 282 to a second input of Δ combiner 321, and the Δ output port of hybrid 223 is coupled by cable 283 to a third input of Δ combiner 321. All the other hybrids (not illustrated) of hybrid array 241 have their Δ output ports coupled to a Δ combiner 321 in a similar manner. Combiner 32′ combines the ′ signals and couples their sum to an output cable 36′.
Each of the other hybrid arrays 242 . . . 24M (only 24M illustrated) of FIG. 1 are connected to an associated pair of sum and difference combiners or beamformers in the same manner. The Mth hybrid array, namely 24M, is illustrated in FIG. 1, together with some of its cables 20, and also with some connection 26 to last column Σ combiner 30M. As so far described, all the columns 161 through 16M ultimately produce a sum signal from a column sum combiner 30 on a cable 34, and a difference signal from a column Δ combiner 32 on a cable 36. Thus, there are M cables 34, and M cables 36, one for each column 16. Elemental phase shifters 18 can be adjusted so that the input signals to column Σ combiners 30 add in-phase for a desired antenna beam pointing direction. Difference signals to column Δ combiner 32 will add in-phase only if cable pairs 26N and 28N are phase matched for all N, provided that the Σ and Δ combiners for each column have identical topologies. First cable 341 and last cable 34M from sum combiners 301 and 30M, respectively, are coupled to individual inputs of a sum-and-difference hybrid designated 381. The outputs from the second (302) and penultimate (30M-1) combiners (not illustrated) are coupled over cables 342 and 34N-1 to separate input ports of a second sum-and-difference hybrid 382. Similarly the third (303) and antepenultimate (30M-2) sum combiners 30 (not illustrated) have their outputs coupled by way of cables 343 and 34M-2, respectively, to a sum-and-difference hybrid 383. Other sum-and-difference hybrids (not illustrated) together with hybrids 381, 382, and 383, form an array 40M of sum-and-difference hybrids. Each hybrid of array 40M receives inputs from a pair of column sum combiners 30 associated with a pair of columns 16, the columns of which are symmetrically disposed to the left and right of the center of array 12.
The sum outputs of the hybrids of hybrid array 40M of FIG. 1 are each separately coupled by way of a cable 44 to a separate input of an azimuth sum combiner 48. For example, hybrid 381 has its Σ output connected by way of a cable 441 to an input of azimuth combiner 48, hybrid 382 has its Σ output connected by a cable 442 to another input of azimuth combiner 48, and hybrid 383 has its Σ output connected by way of a cable 443 to a third input of azimuth sum combiner 48. Azimuth sum combiner combines the Σ signals and produces the combined Σ signal on a cable 50 for application to a processing and display unit illustrated as 70. The Δ outputs of each of sum-and-difference hybrids 38 of hybrid array 40 of FIG. 1 are each separately coupled by way of a cable 46 to separate inputs of an azimuth Δ combiner 52. For example, the Δ output of hybrid 381 is connected by way of a cable 461 to an input of azimuth Δ combiner 52, the Δ output of hybrid 382 is connected to a second input of azimuth Δ combiner 52 by way of a cable 462, and the Δ output of hybrid 383 is connected by way of a cable 463 to yet another input of combiner 52. Combiner 52 combines the Δ signals and applies the combined signals over a cable 54 to processing and display unit 70 of radar unit 10. Another array 41 of sum-and-difference hybrids, each of which is designated as 42 in FIG. 1, is coupled to the array of M column Δ combiners 32 (only combiner 321 is illustrated), in much the same fashion that array 40 of hybrids 38 is coupled to an array of M sum combiners 30. For example, sum-and-difference hybrid 421 receives inputs by way of cables 361 and 36M from first and last column Δ combiners 321 and 32M (not illustrated). Sum-and-difference hybrid 422 is connected by way of cable 362 and 36M-1 to the second and penultimate column Δ combiner 32 (not illustrated), and hybrid 423 has its inputs connected by way of cables 363 and 36M-2 to the third and antepenultimate column Δ combiners 32. Other hybrids 42 of array 41 are connected to other pairs of combiners symmetrically disposed to the left and right about the center of array 12.
The sum outputs of each of sum-and-difference hybrids 42 of array 41 of FIG. 1 are coupled by way of separate cables 56 to separate inputs of an elevation Δ combiner 62. For example, hybrid 421 has its sum output connected by way of a cable 561 to a first input of combiner 62, and the sum outputs of hybrids 422 and 423 are connected by separate cables 562 and 563, respectively, to other inputs of elevation Δ combiner 62. Elevation Δ combiner 62 combines the column Δ signals to produce an elevation Δ signal on a cable 64 for application to processing and display unit 70. The difference (Δ) outputs of sum-and-difference hybrids 42 of hybrid array 41 of FIG. 1 are not used and are terminated. For example, the Δ output of hybrid 421 is coupled by way of cable 581 to a termination 601, and the Δ outputs of hybrids 422 and 423 are coupled by cables 582 and 583 to terminations 602 and 603, respectively.
A transmitter 72 associated with radar system 10 of FIG. 1 is coupled to processing and display unit 70 for timing the signals, for providing appropriate demodulation reference signals, and for other purposes. Also, a transmitter signal is applied to cable 50 of azimuth sum combiner 48, as suggested by dotted lines 74 within processing and display unit 70. The transmitter signals are coupled through azimuth combiner 48 and back through the arrays of hybrids and combiners, which in the context of transmission may act as splitters, to ultimately produce signals at antenna elements 14, which signals are phased in a manner appropriate for directing radiation in a particular direction.
The complexity of the beamforming arrangement of FIG. 1 is apparent. Additional complexity arises because of the amplitude weighting of the signals relative to each other in each column 16, and from column to column, in order to achieve the appropriate beam sidelobe levels for both elevation and azimuth beams. Even if phase shifters 18 are set correctly, assuming equal phase signals arriving at the phase shifters, cumulative phase errors through the combiners and hybrid arrays may adversely affect the performance. In this regard, it should be noted that the actual physical lengths of interconnecting cables such as 201, 202 . . . 20M must be nearly equal for wide bandwidth signals, and some cables such as 26N and 28N must have the same electrical length as well, even though the distances over which the signals must be carried may be less than the physical lengths This in turn tends to create a problem relating to excess cable lengths associated with the shorter paths, which excess cable lengths must be stored out of the way.
FIG. 2A is a simplified block diagram of a monopulse antenna array arrangement as described by Agrawal et al. Elements of FIG. 2A corresponding to those of FIG. 1 are designated by the same reference numerals. Array 12 of FIG. 2A includes a plurality of columns 2161, 2162, 2163 . . . 216M, corresponding generally to columns 16 of FIG. 1. Each column 216 of FIG. 2A includes a vertical array of N antenna elements 14, such as 141, 142, 143 . . . 14N-2, 14N-1, and 14N. Each antenna element 14 of each column 216 is associated with a transmit-receive processor or module (TR Proc). Thus, antenna element 141 of column 2161 is associated with a TR Proc 2181, elemental antenna 142 is associated with TR Proc 2182, and antenna 14N is associated with TR Proc 218N. Structurally, all TR Procs 218 are identical, although their adjustable portions (phase shifters, attenuators and/or switches) may be set differently.
As illustrated in FIG. 2A, each transmit-receive processor 218 has three outputs, designated 219, 220, and 221. For simplicity, the outputs of the TR processors are designated by the same reference numerals as that of the cables to which they are attached. Thus, outputs 2191, 2201 and 2211 of TR Proc 2181 of column 2161 are connected to cables 2191, 2201 and 2211, respectively. In a similar manner, the three outputs of TR Proc 2182 of column 2161 are connected to cables 2192, 2202 and 2212, respectively. The three outputs of TR Proc 218N of column 2161 are separately connected to cables 219N, 220N and 221N. As illustrated in FIG. 2A, the topmost or first TR processor 2181 of column 2162 is seen to be associated with output cables 2191, 2201, and 2211. In column 216M, TR processor 2181 is associated with cables 2191, 2201, and 2211. As in the case of FIG. 1, of course, all the columns 2162 . . . 216N are identical to column 2161.
The arrangement of FIG. 2A includes a Σ beamformer 230, an azimuth Δ beamformer 229, and an elevation Δ beamformer 231. All the cables 219 connected to TR processors 218 of array 12 are gathered in rows and columns in azimuth Δ beamformer 229. For example, all the cables 2191 from TR processors 2181 of all M columns 216 are separately connected to separate inputs located along a top row of beamformer 229. Similarly, all the cables 2192 from all the M TR processors 2182 of all columns 216 of array 12 are gathered and connected to the second row of inputs (not illustrated in FIG. 2A) of azimuth Δ beamformer 229.
FIG. 2B illustrates the connections of TR processors 218 of FIG. 2A to azimuth Δ beamformer 229 of FIG. 2A. In FIG. 2B, the connection face of beamformer 229 is seen in elevation view, with some of the inputs illustrated as dots. The connection face of beamformer 229 contains MXN input ports, one for each TR Proc 218, laid out as M columns and N rows. As can be seen, the upper row of inputs of beamformer 229 for columns 1, 2, 3 . . . M−2, M−1, M are each connected to a cable 2191. The second row of connections of beamformer 229 is to cables 2192, and the bottommost row of connections on the connection face of beamformer 229 receives cables 219N.
Sum beamformer 230 of FIG. 2A is connected to receive cables 220 in a same manner in which beamformer 229 is arranged to receive cables 219. That is, the topmost row of the connection face (not illustrated) of sum beamformer 230 is connected to cables 2201 from all M columns. The second row is connected to cables 2202, and so forth, until the lowermost row is connected to all cables 220N from all M columns. Elevation Δ beamformer 231 is similarly connected to receive cables 221 from all TR Procs 218 of array 12. Azimuth Δ beamformer 229 of FIG. 2A collects all the signals provided over cables 219 to form an azimuth difference signal which is coupled out over a cable 54. In the context of a radar system, cable 54 may be connected to a processor and display unit as described in conjunction with FIG. 1. Similarly, sum beamformer 230 and elevation difference beamformer 231 combine the signals from cables 220 and 221, respectively, to produce combined signals on cables 50 and 64, respectively.
FIG. 3 illustrates one possible arrangement for interconnecting the transmit-receive processors 218 of the arrangement of FIG. 2A, as set forth in the Agrawal et al. patent. In FIG. 3, elements corresponding to those of FIGS. 1 and 2A are designated by the same reference numerals. In FIG. 3, only column 216 and a portion of column 216M are illustrated. Each column of the array, including columns 2161 and 216M, is associated with three individual column beamformers designated 329, 330 and 331. In FIG. 3, azimuth Δ column beamformer 3291 is connected to receive cables 2191, and all other cables 2192, 219N of TR processors 2182-218N of column 216. Column 2161 sum beamformer 3301 receives inputs from cables 2201, 2202, 2202, . . . 220N-2, 220N-1, and 220N. Elevation Δ column beamformer 3311 is connected to receive cable 2211 from TR processor 2181 of column 2161 and cables 2212 . . . 221N from the remaining TR processors 218 of column 2161. Thus, column 2161, and all other columns 216 of array 12, is associated with three column beamformers, one for sum, one for azimuth Δ and the other for elevation Δ. Thus, cables 2201, 2202, 2203 . . . connect from TR processors 2181, 2182, 2183 of column 216M to sum column beamformer 330. Although not illustrated in FIG. 3, column M azimuth difference beamformer 329B is connected to cables 2191, 2192 . . . from the TR processors of column 216M, and column M elevation Δ beamformer 331M is connected to cables 2211, 2212 . . . 221N from the TR processors 218 of column 216M. Each column beamformer 3291-329M of FIG. 3 produces a signal on an output cable 3491-349M. All cables 3491 . . . 349M are connected to corresponding inputs of an array azimuth Δ beamformer 339, which combines the column signals to produce an array azimuth Δ signal on a cable 54. Similarly, elevation Δ column beamformers 3311 . . . 331M each produce a combined output on a corresponding cable 3511 . . . 351M, which are all connected to an array elevation Δ beamformer 341, which combines the signals to produce a combined elevation Δ signal on cable 64. Finally, each sum column beamformer 3301 . . . 330M combines its signals to produce a combined signal on a corresponding cable 3501 . . . 350M. All cables 3501 . . . 350M are connected to corresponding inputs of an array sum beamformer 340, which combines the signals to produce a combined sum signal on a cable 50. Array Σ beamformer 340 of FIG. 3, together with M associated column Σ beamformers 330, may be considered equivalent to sum beamformer 230 of FIG. 2A. Similarly, AZ Δ beamformer 229 of FIG. 2A corresponds to the combination of azimuth Δ beamformer 339 of FIG. 3 with a plurality equal to M of column AZ Δ beamformers 329. Elevation Δ beamformer 231 of FIG. 2A corresponds to the combination of elevation Δ beamformer 341 of FIG. 3 with all M of the column EL Δ beamformers 331.
More recent array antenna arrangements may generate more than three separate beams. In general, each beam is associated with a port of the beamformer. An overlap beamformer feeds at least some, and often most, elements of an antenna array with energy for multiple beams, and the number of beams may exceed three. Inexpensive and reliable interconnections of the beamformer(s) with the antenna elements are desirable, but the topology of the connections tends to make conventional approaches tends to require a great deal of hand work and checking of connections against drawings. This hand work, in turn, tends to reduce the reliability of the connections, and increases the cost of the connections.
Improved beamformers and interconnection arrangements therefor are desired.